Endogenous opioids are believed to be involved in the modulation of pain perception, in mood and behavior, learning and memory, diverse neuroendocrine functions, immune regulation and cardiovascular and respiratory function. Opioids also have a wide range of therapeutic utilities, such as treatment of opiate and alcohol abuse, neurological diseases, neuropeptide or neurotransmitter imbalances, neurological and immune system dysfunctions, graft rejections, pain control, shock and brain injuries.
There are believed to be three types of opiate receptors, namely δ, κ and μ. Genes encoding these three main receptor types now have been cloned. Sequencing of the cloned opioid receptor genes has revealed a substantial degree of amino acid homology between different receptor types (Meng et al., PNAS USA 90: 9954-9958 (1993); Thompson et al., Neuron 11: 903-913 (1993); Evans et al., Science 258: 1952-1955 (1992); and Kieffer et al., PNAS USA 89: 12048-12052 (1992)), which explains the tendency of opioid receptor ligands, even those reported to be selective, to bind to more than one type of opioid receptor. Based on differences in the binding profiles of natural and synthetic ligands, subtypes of opioid receptors have been suggested, including μ1 and μ2 (Pasternak et al., Life Sci. 38: 1889-1898 (1986)) and κ1 and κ2 (Zukin et al., PNAS USA 85: 4061-4065 (1988)). Different subtypes of a given type of opioid receptor may co-exist in a single cell (Evans et al. (1992), supra; and Kieffer et al. (1992), supra).
The μ opioid receptor in the brain appears to mediate analgesia (Kosterlitz et al., Br. J. Pharmacol. 68: 333-342 (1980)). It is also believed to be involved with other undesirable effects, such as respiratory depression (Ward et al., Soc. Neurosci. Symp. 8: 388 (abstract) (1982)), suppression of the immune system (Plotnikoff et al., Enkephalins and Endorphins: Stress and the Immune System, Plenum Press, NY (1986); Yahya et al., Life Sci. 41: 2503-2510 (1987)) and addiction (Roemer et al., Life Sci. 27: 971-978 (1981)). Its side effects in the periphery include inhibition of intestinal motility (Ward et al., Eur. J. Pharmacol. 85: 163-170 (1982)) and secretion in the small intestine (Coupar, Br. J. Pharmacol. 80: 371-376 (1983)).
δ-opioid receptors also mediate analgesic but are not involved in addiction. They may have an indirect role in immune suppression.
There appears to be a single binding site for agonists and antagonists in the ligand-binding domain of δ receptors. Thus, the “message domain” of δ-agonists and δ-antagonists probably presents a similar low energy conformer in order to fit the receptor cavity. The minimum size of that “message domain” constitutes the dimensions of a dipeptide (Temussi et al., Biochem. Biophys. Res. Commun. 198: 933-939 (1994); Mosberg et al., Lett. Pept. Sci. 1: 69-72 (1994); and Salvadori et al., J. Med. Chem. 42: 3100-3108 (1997)), which has a specific spatial geometry in solution (Bryant et al., Trends Pharmacol. Sci. 18: 42-46 (1998); Bryant et al., Biol. Chem. 378: 107-114 (1997); Crescenzi et al., Eur. J. Biochem. 247: 66-73 (1997); and Guerrini et al., Bioorg. Med. Chem. 6: 57-62 (1998)) as seen in the crystallographic evidence for TIPP analogues (Flippen-Anderson et al., J. Pept. Res. 49: 384-393 (1997)) and N,N(Me)2-Dmt-Tic-OH.
The Dmt-Tic pharmacophore represents a distinct class of δ-opioid antagonists (Salvadori et al., Mol. Med. 1: 678-689 (1995); Bryant et al. (1998), supra; and Lazarus et al., Drug Dev. Today 1998: 284-294). Observations of differences between the δ opioid receptor binding of Dmt-Tic peptides and their Tyr-Tic cognates (Salvadori et al. (1995), supra; Lazarus et al. (1998), supra; and Lazarus et al., Int'l Symp. on Peptide Chem. and Biol., Changchung, PRC (1999)) indicates that Dmt assumes a predominant role in the alignment or anchoring of the peptide within δ, μ and κ opioid receptor binding sites (Bryant et al. (1998), supra; and Bryant et al. (1997), supra; Crescenzi et al. (1997), supra; and Guerrini et al. (1998), supra) or affects the conformation of the dipeptide antagonists in solution (Bryant et al. (1997), supra; and Crescenzi et al. (1997), supra). Furthermore, observations of differences between the spectra of activity exhibited by the Tyr-Tic cognates of certain Dmt-Tic peptides (Schiller et al., PNAS USA 89: 11871-11875 (1992); Schiller et al., J. Med. Chem. 36: 3182-3187 (1993); Schiller et al., Peptides Hodges and Smith, eds., ESCOM (1994); pp. 483-486; Temussi et al. (1994), supra; Mosberg et al. (1994), supra; Salvadori et al. (1995), supra; Lazarus et al. (1998), supra; and Lazarus et al. (1999), supra) and the corresponding Dmt-Tic peptides suggests that the C-terminal “address” portion of the peptide can influence the “message domain.”
Recently, cyclic peptides and di- and tri-peptides comprising the pharmacophore Dmt-Tic have been developed and have been shown to exhibit high selectivity, affinity and potency for the δ-opioid receptor. Such peptides have been shown to function as agonists, partial agonists, antagonists, partial antagonists or mixed antagonists/agonists for opioid receptors (see Lazarus et al., U.S. Pat. No. 5,780,589, and Schiller, U.S. Pat. No. 5,811,400).
A variety of modifications to the Tic residue differentially changes receptor selectivity, including alterations in its electronic configuration and chirality, as well as its replacement by heteroaliphatic/heteroaromatic nuclei or D-Phe (Santagada et al., Med. Chem. Lett., 10, 2745-2748 (2000); Pagé et al., Bioorg. Med. Chem. Lett., 10, 167-170 (2000); Salvadori et al., Mol. Med., 1: 678-689 (1995); Balboni et al., Peptides, 21: 1663-1671 (2000); and Capasso et al., FEBS Lett., 417: 141-144 (1997)). Changes wrought by altering the distance to a third aromatic center (“Y”) at the C-terminus by an interposed sequence, a spacer (“X”), induces profound changes in the affinity, selectivity and bioactivity of a ligand (Capasso et al., FEBS Lett., 417: 141-144 (1997); Salvadori et al., J. Med. Chem., 42: 5010-5019 (1999); Pagé et al., J. Med. Chem., 44: 2387-2390 (2001)).
The uniqueness of the δ receptor has led to the use of moderately δ-selective alkaloid antagonists in clinical trials, such as for the amelioration of the effects of alcoholism (Froehlich et al., Alcohol. Clin. Exp. Res. 20: A181-A186 (1996)), the treatment of autism (Lensing et al., Neuropsychobiol. 31: 16-23 (1995)), and Tourette's syndrome (Chappell, Lancet 343: 556 (1994)). The δ-opiate antagonist naltrindole (Portoghese et al., Eur. J. Pharm. 146: 185-186 (1998)) has been shown to inhibit the reinforcing properties of cocaine (Menkens et al., Eur. J. Pharm. 219: 346-346 (1992)), to moderate the behavioral effects of amphetamines (Jones et al., J. Pharmacol. Exp. Ther. 262: 638-645 (1992)), and to suppress the immune system (Jones et al. (1992), supra) for successful organ transplantation (House et al., Neurosci. Lett. 198: 119-122 (1995)) in animal models (Arakawa et al., Transplant Proc. 24: 696-697 (1992); Arakawa et al., Transplant 53: 951-953 (1992); and Arakawa et al., Transplant. Proc. 25: 738-740 (1993)). The same effects also have been shown for 7-benzylspiroindanylnaltrexone (Lipper et al., Eur. J. Pharmacol. 354: R3--R5 (1998)).
The intractable membrane barriers, such as the blood-brain barrier (BBB), must be circumvented in order for peptide antagonists to express activity in vivo (Ermisch et al., Physiol. Rev. 73: 489-527 (1993)). The requisite physicochemical properties of compounds capable of passing through this barrier include low molecular weight (<800 Da) and high octanol-water coefficient characteristics.
In view of the above, the present invention seeks to provide more potent δ-opioid antagonists and δ-opioid antagonists with high dual binding affinity and biological activity toward δ-opioid and μ-opioid receptors.
In addition to the above, one of the major chemical determinants for the inhibition of hMDR-1 involves the presence of strong hydrophobic substituents necessary for lipid solubility (Ford et al., Cancer Res. 50: 1748-1756 (1990); Zamora et al., Mol. Pharmacol. 33: 454-462 (1988); and Nogae et al., Biochem. Pharmacol. 38: 519-527 (1989)) as constituted by saturated and aromatic rings, as well as a tertiary nitrogen (Zamora et al. (1988), supra; Ramu et al., Int. J. Cancer 43: 487-491 (1989); and Pearce et al., PNAS USA 86: 5128-5133 (1989)). In addition to a role in drug resistance, the human Pgp-1 protein is expressed in a variety of normal human tissues and plays an important physiological role in maintenance of the BBB (Schinkel et al., J. Clin. Invest. 97: 2517-2524 (1996); Schinkel et al., Int. J. Clin. Pharmacol. Ther. 36: 9-13 (1998); and Jonker et al., Br. J. Pharmacol. 127: 43-50 (1999)). In the BBB, Pgp activity in apical membranes of the capillary endothelial cells functions to at least partially exclude a wide variety of hydrophobic toxicants from the brain (Zamora et al. (1988), supra; Schinkel et al. (1996), supra; and Jonker et al., Br. J. Pharmacol. 127: 43-50 (1999)). These include some drugs with central nervous system activities, such as certain opiate alkaloids and analogues thereof (Callaghan et al., J. Biol. Chem. 268: 16059-16064 (1993)). Thus, the present invention also seeks to provide inhibitors of hMDR-1.
These and other objects of the present invention, as well as additional inventive features, will be apparent to the ordinarily skilled artisan from the detailed description provided herein.